From f93bac061c59a6efab309b43eb893bf041c93ee1 Mon Sep 17 00:00:00 2001
From: Peter Williams <peter@newton.cx>
Date: Sun, 30 Jan 2022 11:12:58 -0500
Subject: [PATCH] Start implementing the MATH table

---
 src/font.rs       |   6 ++
 src/lib.rs        |   1 +
 src/math.rs       | 155 ++++++++++++++++++++++++++++++++++++++++++++++
 src/otl/shared.rs |   2 +-
 4 files changed, 163 insertions(+), 1 deletion(-)
 create mode 100644 src/math.rs

diff --git a/src/font.rs b/src/font.rs
index aeb17b5..762ff71 100644
--- a/src/font.rs
+++ b/src/font.rs
@@ -13,6 +13,7 @@ use super::{
     hhea::*,
     hmtx::*,
     hvar::*,
+    math::*,
     maxp::*,
     name::*,
     os2::*,
@@ -329,6 +330,11 @@ pub trait TableProvider<'a> {
     fn gpos(&self) -> Option<Gpos<'a>> {
         Some(Gpos::new(self.table_data(GPOS)?, self.gdef()))
     }
+
+    /// Returns the mathemetical typesetting table.
+    fn math(&self) -> Option<Math<'a>> {
+        Some(Math::new(self.table_data(MATH)?))
+    }
 }
 
 impl<'a> TableProvider<'a> for FontRef<'a> {
diff --git a/src/lib.rs b/src/lib.rs
index 8203630..d5e473d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -14,6 +14,7 @@ pub mod head;
 pub mod hhea;
 pub mod hmtx;
 pub mod hvar;
+pub mod math;
 pub mod maxp;
 pub mod name;
 pub mod os2;
diff --git a/src/math.rs b/src/math.rs
new file mode 100644
index 0000000..10a5ec6
--- /dev/null
+++ b/src/math.rs
@@ -0,0 +1,155 @@
+//! Mathematical typesetting table.
+
+use super::otl::Coverage;
+use super::parse_prelude::*;
+
+/// Tag for the `math` table.
+pub const MATH: Tag = Tag::new(b"MATH");
+
+/// Mathematical typesetting table.
+///
+/// <https://docs.microsoft.com/en-us/typography/opentype/spec/math>
+///
+/// The math constants and math glyph information subtables are not (yet)
+/// implemented.
+#[derive(Copy, Clone)]
+pub struct Math<'a>(Buffer<'a>);
+
+impl<'a> Math<'a> {
+    /// Creates a new math table from a byte slice containing the table data.
+    pub fn new(data: &'a [u8]) -> Self {
+        Self(Buffer::new(data))
+    }
+
+    /// Returns the major version.
+    pub fn major_version(&self) -> u16 {
+        self.0.read(0).unwrap_or(0)
+    }
+
+    /// Returns the minor version.
+    pub fn minor_version(&self) -> u16 {
+        self.0.read(2).unwrap_or(0)
+    }
+
+    /// Returns the MathVariants subtable.
+    pub fn variants(&self) -> Option<MathVariants> {
+        let offset = self.0.read_offset16(8, 0)?;
+        Some(MathVariants { math: self, offset })
+    }
+}
+
+/// Mathematical variants subtable.
+///
+/// <https://docs.microsoft.com/en-us/typography/opentype/spec/math#mathvariants-table>
+#[derive(Copy, Clone)]
+pub struct MathVariants<'a> {
+    math: &'a Math<'a>,
+    offset: u32,
+}
+
+impl<'a> MathVariants<'a> {
+    /// Returns the minimum overlap of connecting glyphs during glyph
+    /// construction.
+    pub fn min_connector_overlap(&self) -> UfWord {
+        self.math.0.read(self.offset as usize).unwrap_or(0)
+    }
+
+    /// Returns the number of glyphs for which information is provided for
+    /// vertically growing variants.
+    pub fn vert_glyph_count(&self) -> u16 {
+        self.math.0.read(self.offset as usize + 6).unwrap_or(0)
+    }
+
+    /// Returns the number of glyphs for which information is provided for
+    /// horizontally growing variants.
+    pub fn horiz_glyph_count(&self) -> u16 {
+        self.math.0.read(self.offset as usize + 8).unwrap_or(0)
+    }
+
+    /// Returns the coverage table associated with vertically growing glyphs.
+    pub fn vert_glyph_coverage(&self) -> Option<Coverage> {
+        let offset = self
+            .math
+            .0
+            .read_offset16(self.offset as usize + 2, self.offset)?;
+        Some(Coverage::new(self.math.0, offset))
+    }
+
+    /// Returns the coverage table associated with horizontally growing glyphs.
+    pub fn horiz_glyph_coverage(&self) -> Option<Coverage> {
+        let offset = self
+            .math
+            .0
+            .read_offset16(self.offset as usize + 4, self.offset)?;
+        Some(Coverage::new(self.math.0, offset))
+    }
+
+    /// Returns information about how to a construct vertically growing glyph,
+    /// based on its coverage index.
+    pub fn vert_glyph_construction(&self, coverage_index: u16) -> Option<MathGlyphConstruction> {
+        let offset = self.offset as usize + 10 + 2 * coverage_index as usize;
+        let offset = self.math.0.read_offset16(offset, self.offset)?;
+        Some(MathGlyphConstruction {
+            math: self.math,
+            offset: offset,
+        })
+    }
+
+    /// Returns information about how to a construct horizontally growing glyph,
+    /// based on its coverage index.
+    pub fn horiz_glyph_construction(&self, coverage_index: u16) -> Option<MathGlyphConstruction> {
+        let offset = self.offset as usize
+            + 10
+            + 2 * self.vert_glyph_count() as usize
+            + 2 * coverage_index as usize;
+        let offset = self.math.0.read_offset16(offset, self.offset)?;
+        Some(MathGlyphConstruction {
+            math: self.math,
+            offset: offset,
+        })
+    }
+}
+
+/// Mathematical glyph construction subtable.
+///
+/// <https://docs.microsoft.com/en-us/typography/opentype/spec/math#mathvariants-table>
+///
+/// The "glyph assembly" subtable is not (yet) implemented.
+#[derive(Copy, Clone)]
+pub struct MathGlyphConstruction<'a> {
+    math: &'a Math<'a>,
+    offset: u32,
+}
+
+impl<'a> MathGlyphConstruction<'a> {
+    /// Returns the number of growing variants for this glyph.
+    pub fn variant_count(&self) -> u16 {
+        self.math.0.read(self.offset as usize + 2).unwrap_or(0)
+    }
+
+    /// Return the growing variants associated with this glyph.
+    pub fn variants(&self) -> Option<Slice<'a, MathGlyphVariantRecord>> {
+        self.math
+            .0
+            .read_slice(self.offset as usize + 4, self.variant_count() as usize)
+    }
+}
+
+/// Information about a math glyph variant.
+#[derive(Copy, Clone, Debug)]
+pub struct MathGlyphVariantRecord {
+    /// The variant glyph
+    pub variant_glyph: GlyphId,
+    /// The advance width/height of the variant, in the direction of this
+    /// record's associated table.
+    pub advance_measurement: UfWord,
+}
+
+impl ReadData for MathGlyphVariantRecord {
+    unsafe fn read_data_unchecked(buf: &[u8], offset: usize) -> Self {
+        Self {
+            variant_glyph: GlyphId::read_data_unchecked(buf, offset),
+            advance_measurement: UfWord::read_data_unchecked(buf, offset + 2),
+        }
+    }
+}
diff --git a/src/otl/shared.rs b/src/otl/shared.rs
index 24f0f1d..f1a00fb 100644
--- a/src/otl/shared.rs
+++ b/src/otl/shared.rs
@@ -13,7 +13,7 @@ pub struct Coverage<'a> {
 }
 
 impl<'a> Coverage<'a> {
-    pub(super) fn new(data: Buffer<'a>, offset: u32) -> Self {
+    pub(crate) fn new(data: Buffer<'a>, offset: u32) -> Self {
         Self { data, offset }
     }
 
